As is well known, digital processing techniques of television signals have rapidly advanced, and various types of apparatuses now used are of digital type. This tendency to use digital circuitry applies also to a superimposed edge generation apparatus when high performance digital processing is required.
FIG. 1 shows a conventional digital superimposed edge generation apparatus. A digital superimpose key signal S.sub.K generated by a predetermined inserted image is supplied to a V (vertical) direction edge addition circuit 11. The edge addition circuit 11 adds an edge of a predetermined width in the vertical direction of the key signal. The V direction edge addition circuit 11 produces a V direction edge signal E.sub.V and the superimpose key signal S.sub.K which is delayed by a predetermined time. The delayed superimpose key signal S.sub.K from circuit 11 is supplied to an H (horizontal) direction edge addition circuit 12. The edge addition circuit 12 adds an edge of a predetermined width (determined from the superimpose key) in the horizontal direction of the key signal. The circuit 12 produces an H direction edge signal E.sub.H. The H direction edge signal E.sub.H and the vertical edge signal E.sub.V are supplied to a correction circuit 13. The correction circuit 13 multiplies the edge signals E.sub.H and E.sub.V so as to correct the width of the edge at a corner of the key signal to a predetermined width. In a digital superimposed edge generation apparatus of this type, processing in the H and V directions are performed independently of each other and the relationship between the edges in the H and V directions is corrected afterwards. Therefore, it is possible to add an edge of a predetermined width to a key signal of H or V direction. However, correction procedures become complex for a key signal of a recessed shape, and it is difficult to add an edge of a predetermined width to such a signal.
When an edge is to be added to a circular key signal K as shown in FIG. 2, the edge width becomes narrower in directions l1 and l2 of 45.degree. than in the H and V directions. This occurs because the edge signals in the H and V directions are calculated first, and then the edge signals in the l1 and l2 directions are obtained by multiplying the obtained signals. Thus, in a conventional apparatus, it is difficult to generate an edge of a constant width depending upon the shape of a key signal, resulting in an unnatural appearance.
The present invention has been made in consideration of this and has for its object to provide a digital superimposed edge generation apparatus which is capable of generating an edge of a constant width irrespective of the shape of a key signal.